Imagine baking a chocolate cake with bakers chocolate and no sweetener. You would have a cake with no flavor, yet somehow manage to leave a nasty taste in your mouth.
The same can be said of characters who haven't had been fully developed and spiced up a bit.
A hero or heroine with no history or depth will leave the same acrid taste in your reader's mouth. He will open the book to sample it and never sink his teeth into it again.
Before writing your book create an outline of each character.
This should include:
likes and dislikes
hidden strengths and talents
Then take a few pages and develop a full profile on your character, even if many of these details will never be seen in your book.
Why is this important?
If you don't know your characters, your readers never will.
Worse yet they will come off as flat and uninteresting.
Ex. Matt went to school. He got into a fight with Peter and was sent into the principles office. They later made up and learned the meaning of getting along.
Matt and Peter were at it again. Julie covered her eyes, as the other students squirmed to get a better look.
Principle Snider stormed down the hall with murder in his eyes. This was the last straw.
Do you get a sense that there is a history with these two boys? What about their relationship with the principle? Why would Julie cover her eyes? Is she a longtime friend of one or both of them?
Create a rich history for each character then develop the connections between them. It will not only develop your characters more fully but may even fill your mind with storylines and potential conflicts.